1. Field of the Invention
The present invention relates to a method and apparatus for controlling the size of ink dots produced by a printing device and, more particularly, to a method and apparatus for controlling the spread of ink on an outer surface of an ink transfer printing device.
2. Description of the Related Art
Image quality can be improved not only by increasing resolution, but also by using halftone techniques. The information content of a halftone image goes beyond resolution and includes different dot sizes and maybe even different shapes of the ink dots. As an example, a 150 dot per inch (dpi) image with 16 dot sizes will have a quality comparable to a 600 dpi image having a single dot size.
Ink jet printers produce drops which are ejected from an orifice towards a printing media. Several techniques are known for controlling the volume of ink drops produced by an ink jet printer. For example, the volume of ink drops has been controlled in piezoelectric drop-on demand printers by varying pulse height or pulse width of the applied electrical energy. Each pulse of energy produces a single drop of ink. The volume of ink drops has also been controlled by ejecting a group of drops which merge together to form a larger drop. However, each individual drop ejected has a common volume which is determined by the orifice size.
The present invention relates to printing techniques in which ink flows onto an ink transfer surface through orifices (e.g., ink transfer printing). In contrast, ink jet printers spray or jet ink through nozzles towards a printing media. As a result, the ink drop volume control techniques used with ink jet printers are not suitable for controlling ink dot size in ink transfer printing devices. Hence, novel techniques for controlling ink dot size in ink transfer printing devices are needed.
Additional background information on known printing techniques is contained in "Computer Graphics--Technology and Applications," Vol. II-- "Output Hardcopy Devices," by Robert C. Durbeck and Sol Sherr, San Diego 1988.